https://bugs.llvm.org/show_bug.cgi?id=52195
Bug ID: 52195
Summary: missed optimization: external pointer used as integer
should fold into immediate operand
Product: new-bugs
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
When a program converts an external pointer to an integer, LLVM won't use the
resulting value as an immediate operand.
Example 1: https://godbolt.org/z/hnKoWcvGc
#include <stdint.h>
extern char addr;
void g(unsigned);
void f(unsigned x) {
unsigned val = (unsigned)(uintptr_t)&addr;
return g(x + val);
}
Output:
f: # @f
mov eax, offset addr
add edi, eax
jmp g # TAILCALL
Desired output:
f: # @f
add edi, offset addr
jmp g # TAILCALL
Example 2: https://godbolt.org/z/46vEEjbnY
#include <stdint.h>
extern char addr;
void g(unsigned long);
void f(unsigned long x) {
unsigned long val = (unsigned)(uintptr_t)&addr;
return g(x + val);
}
Output:
f: # @f
mov eax, offset addr
mov eax, eax
add rdi, rax
jmp g # TAILCALL
Desired output:
f: # @f
add rdi, offset addr
jmp g
These examples may seem unusual, but I am considering a novel JIT design where
I would use the compiler's relocatable output as JIT input, and use the emitted
relocations to tell me where to "patch in" my runtime values (which will be
integers, not actually pointers).
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